Dairy cattle, which are maintained under close supervision and in a relatively confined area are generally bred by artificial insemination. Inasmuch as the time during which a cow is in heat is relatively short, it is of considerable importance to be able to detect a cow in heat as early as possible and to be able to single her out and separate her for artificial insemination. It is recognized that a cow in heat will permit other cows to mount her, generally during the early stages of a heat. Various detecting devices have been provided which indicate that a cow has been mounted, but such devices may be triggered by a single mounting and are not reliable indicators that a cow is actually in heat. I have determined that when a cow is in heat and submits to repeated mountings from the rest of the cows, there is caused an extreme irritation on her tail head which produces a localized zone there, warmer then the rest of the body.